Liquid gauging is the determination or measurement of a quantity of liquid in a container. The quantity of liquid in a container can be defined or expressed in different units of measure, such as volume, weight, mass and height of the liquid surface in the container. As an example, in aircraft applications, aircraft fuel is contained in a plurality of fuel tanks on the aircraft. Typically, it is the mass of fuel in a tank that determines in large measure the flight distance for the aircraft. Although a measure of the fuel volume and weight may also be useful, ultimately it is an accurate determination of the mass of fuel in the tanks that is of primary interest.
The performance of any liquid gauging system is significantly dependent on the number and locations of the various liquid sensors or probes. The sensors can include any number of devices that detect a characteristic of the liquid related to volume, such as the height of the liquid surface. Height sensors include ultrasonic sensors and pressure sensors. Although these sensors do not measure surface height directly, they can be used to measure a parameter that is directly related to surface height. For example, an ultrasonic sensor can be used to measure the time delay between transmitting an acoustic energy pulse towards the liquid surface and detecting an acoustic echo reflected from the surface back toward the acoustic transmitter. Some gauging systems, especially fuel gauging systems, use elongated probes that are mounted within the liquid and detect liquid height as a function of the percentage of the probe that is immersed in the liquid.
In most liquid gauging systems, system performance and accuracy depends on the use of a sufficient number of probes, both for redundancy to increase reliability, and sufficient coverage to assure that at least one probe or sensor is capable of detecting the liquid surface. This is particularly so in dynamic systems such as aircraft in which the fuel tank can be subjected to a wide range of pitch/roll attitudes.
The number of probes/sensors and the often complex structure and geometry of the tank make the process of finding the optimal sensor/probe locations a formidable and time consuming task. Since there is rarely a linear relationship between surface height and volume (due, for example, to complex tank geometry, internal structures and attitude variations), one known approach is to use probe profiling to account for varying tank geometry at different fuel heights. Look-up tables are commonly used to compensate the probe reading at different attitudes. Regardless of the technique used to improve gauging accuracy, location of the sensors or probes will always be important. Currently, this involves a significant reliance on the skill and experience of the design engineer, and many times the final placement of the sensors and probes is as much a function of intuition and experience as it is on empirical data. This follows from the observation that in order to try out every possible combination of sensor/probe locations within a complex tank structure would involve an extraordinary amount of design time and testing. This problem is exacerbated as the number of probes and tank geometry complexity increases. The entire process must also be started anew when design changes are made to the tank structure or the type and number of probes/sensors are changed.
The objectives exist, therefore, for a wholly different approach to optimizing the location of liquid gauging sensors and probes with respect to the liquid container that will reduce the design labor and time requirements, and is flexible to re-compute the optimal locations when design changes are implemented, such as changes in the container geometry or the number and type of sensors and probes.